Willwood Formation paleosols are ranked on a scale of 0 to 5 on the basis of their relative maturity (= relative time required to form). In the lateral dimension, the least mature soils were developed more proximal to ancient channel belts, whereas the more mature paleosols formed in areas more distant to channel belts. Quantitative study shows that both mammalian taxonomic composition and taphonomic completeness vary systematically with the maturity of these paleosols. Species-level differences in taxonomic composition are identified for pedofacies sequences located at the 442-m and 546-m levels of the Willwood Formation. At 442 m, Cantius frugivorus and Hyopsodus sp., cf. H. minor account for practically all of the adapiform primate and hyopsodontid condylarth faunas in stage 3 to 4 paleosols (which are distally located with respect to the ancient channel belt). Laterally adjacent and stratigraphically equivalent stage 1 to 2 paleosols (proximally located with respect to the ancient channel belt), are instead dominated by Cantius sp. nov. and Hyopsodus sp., cf. H. miticulus. Intermediate proportions of these taxa occur at localities in paleosols of intermediate maturity (stage 2 to 3 paleosols) at the 442-m level. At 546 m, the otherwise relatively rare species Hyopsodus powellianus makes up nearly 50 percent of the hyopsodontid fauna at some localities developed in stage 1 paleosols; elsewhere in this pedofacies the species Hyopsodus minor and H. lysitensis make up the overwhelming majority of the Hyopsodus. Also at 546 m, the adapiform primates Cantius abditus and “Copelemur”feretutus exhibit reversals in relative abundance from proximal to distal localities across the pedofacies; Cantius is more abundant in proximal localities and “Copelemur” is dominant in distal localities. Ordinal-level differences in taxonomic composition were detected at localities in two distinct pedofacies lying at or slightly above Biohorizon C (= “Graybullian-Lysitean” boundary). There, Condylarthra and Artiodactyla are more common in immature (stages 1 to 2) than mature (stage 4) paleosols, whereas the reverse is true for Primates, Carnivora, Rodentia, and Perissodactyla. Lateral controls on completeness of skeletal elements, as related to lateral variation in sedimentation rate, are also evident. Proportions of less complete skeletal elements are considerably higher at localities developed in mature paleosols, where sedimentation rates were low. These findings underscore the inherent relatedness of geographic distribution of taxa, taphonomy, and sedimentology and suggest that intrabasinal differences in microhabitat had a significant effect on the local taxonomic composition of the Willwood mammalian fauna.

The first Eocene vertebrate assemblage known from the Great Basin, the Elderberry Canyon Local Fauna, occurs in rocks referred to the Sheep Pass Formation near Ely, Nevada. Approximately 40 taxa are now known, including small anuran amphibians, small reptiles, birds, and mammals. The mammalian component consists of: the insectivorans Apatemys bellus, Pantolestes longicaudus, a tiny apternodont, at least one nyctitheriid, and at least four other taxa representing dormaalid and/or erinaceid erinaceomorphs; an epoicotheriid palaeanodont, cf. Tetrapassalus mckennai; the primates Notharctus tenebrosus, Trogolemur myodes, and two species of uintasoricines; the rodents Reithroparamys delicatissimus, R. cf. R. huerfanensis, Sciuravus sp., Microparamys sp., Pauromys sp., Mattimys sp., and two new genera; the hyaenodont Sinopa minor; two viverravid carnivores including Viverravus; the condylarth Hyopsodus paulus; the perissodactyls Hyrachyus modestus, Hyrachyus affinis, Helaletes nanus, Isectolophus latidens, and a new genus and species; and the artiodactyl Antiacodon pygmaeus. Greatest faunal similarity is with the Black’s Fork Member (or lower), Bridger Formation, and other early Bridgerian localities such as Powder Wash in the Douglas Creek Member of the Green River Formation in northeastern Utah. The age of the Elderberry Canyon Local Fauna can confidently be called early Bridgerian. The Elderberry Canyon Fauna is preserved in carbonate rocks believed to have been deposited in a shallow, warm, heavily vegetated, permanent, hardwater lake. The mammals lived on marshy wetland terrain adjacent to the lake, although some faunal elements may have been transported in from more distant habitats.